In an idling stopped vehicle which restarts an engine many times, since power of a battery is consumed by in-vehicle equipment such as an AV system, an air conditioner, etc. during a period of temporarily stoppage of the engine, battery voltage drop during cranking (engine starting) becomes stricter than before. By using a step-up/down switching regulator which holds an output voltage when an input voltage (battery voltage) drops, the in-vehicle equipment can be normally operated even when the battery voltage drops greatly during the cranking.
For this reason, the demand for step-up/down switching regulators is increasing in the in-vehicle equipment market.
The configuration and operation of a typical step-up/down switching regulator will be described below. FIG. 15 is a diagram illustrating the configuration of a typical step-up/down switching regulator.
The step-up/down switching regulator shown in FIG. 15 includes MOS transistors Q11 and Q12, which are step-down switches, an inductor L11, MOS transistors Q13 and Q14, which are step-up switches, an output capacitor C11, resistors R11 to R14 and a control part CNT11.
The control part CNT11 monitors an output voltage VOUT by an output of a voltage dividing circuit configured with the resistors R11 and R12 and monitors a battery voltage VBAT, which is an input voltage, by an output of a voltage dividing circuit configured with the resistors R13 and R14.
When the battery voltage VBAT is larger than a first predetermined value A1, the control part CNT11 selects a step-down mode (see FIG. 16). In the step-down mode, the control part CNT11 turns on/off the MOS transistors Q11 and Q12 in response to the output voltage VOUT, always turns off the MOS transistor Q13, and always turns on the MOS transistor Q14. As a result, a first switch voltage VSW1, which is a voltage of a connection node between the MOS transistors Q11 and Q12, and a second switch voltage VSW2, which is a voltage of a connection node between the MOS transistors Q13 and Q14, give an output as shown in FIG. 17A.
When the battery voltage VBAT is equal to or smaller than the first predetermined value A1 and is larger than a second predetermined value A2, the control part CNT11 selects a step-up/down mode (see FIG. 16). In the step-up/down mode, the control part CNT11 turns on/off the MOS transistors Q11 and Q12 in response to the output voltage VOUT and turns on/off the MOS transistors Q13 and Q14 in response to the output voltage VOUT. As a result, the first switch voltage VSW1, which is the voltage of the connection node between the MOS transistors Q11 and Q12, and the second switch voltage VSW2, which is the voltage of the connection node between the MOS transistors Q13 and Q14, give an output as shown in FIG. 17B.
When the battery voltage VBAT is equal to or smaller than the second predetermined value A2, the control part CNT11 selects a step-up mode (see FIG. 16). In the step-up mode, the control part CNT11 always turns on the MOS transistor Q11, always turns off the MOS transistor Q12, and turns on/off the MOS transistors Q13 and Q14 in response to the output voltage VOUT. As a result, the first switch voltage VSW1, which is the voltage of the connection node between the MOS transistors Q11 and Q12, and the second switch voltage VSW2, which is the voltage of the connection node between the MOS transistors Q13 and Q14, give an output as shown in FIG. 17C.
An average current of the inductor L11 in the step-up/down mode and the step-up mode is larger than that in the step-down mode. For this reason, heat generation in the step-up/down mode and the step-up mode is greater than that in the step-down mode, which results in poor efficiency. Therefore, it is desirable to switch between the step-down mode to the step-up/down mode as little as possible even when the battery voltage VBAT is lowered. That is, it is desirable to set the first predetermined value A1 to be as small as possible within a range in which a desired output voltage VOUT is obtained.
However, in the step-up/down switching regulator shown in FIG. 15, a switching frequency of the MOS transistors Q11 and Q12 is a constant value in the step-down mode, thereby limiting the maximum on-duty ratio of the MOS transistor Q11. In addition, the first predetermined value A1 cannot be sufficiently lowered because of the limited maximum on-duty ratio.
A conventional DC-DC converter also has the same problems as the step-up/down switching regulator shown in FIG. 15.